chore(iast): improve iast report performance (#13300)

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
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[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Author: avara1986

ddtrace/appsec/_iast/_iast_request_context.py

@@ -46,8 +46,8 @@ def _create_and_attach_iast_report_to_span(req_span: "Span", existing_data: Opti
         report_data._merge(previous_data)
 
     if report_data is not None:
-        report_data.build_and_scrub_value_parts()
-        req_span.set_tag_str(IAST.JSON, report_data._to_str())
+        data = report_data.build_and_scrub_value_parts()
+        req_span.set_tag_str(IAST.JSON, report_data._to_str(data))
     _set_metric_iast_request_tainted()
     base._set_span_tag_iast_request_tainted(req_span)
     _set_span_tag_iast_executed_sink(req_span)

ddtrace/appsec/_iast/reporter.py

@@ -342,7 +342,7 @@ def get_unredacted_value_parts(self, evidence_value: str, ranges: List[dict], so
 
         return value_parts
 
-    def _to_str(self) -> str:
+    def _to_str(self, dict_data=None) -> str:
         """
         Converts the IAST span reporter to a JSON string.
 
@@ -359,4 +359,6 @@ def default(self, obj):
                     return origin_to_str(obj)
                 return json.JSONEncoder.default(self, obj)
 
+        if dict_data:
+            return json.dumps(dict_data, cls=OriginTypeEncoder)
         return json.dumps(self._to_dict(), cls=OriginTypeEncoder)